Transflective type LCD device having excellent image quality

ABSTRACT

An LCD device has a reflective area that reflects light incident from a polarizing film side using a reflection film, and a transmissive area that transmits light of a backlight incident from a TFT substrate side. The drive voltages of the reflective area and transmissive area are Vr and Vt, the black voltage in the reflective area is Vr (K), the black voltage in the transmissive area is Vt (K). The reflectance R, the transmittance T, characteristics of R with respect to drive voltage [Vr (K)−Vr] and characteristics of T with respect to drive voltage [Vt−Vt (K)] substantially match each other.

CROSS-REFERENCE TO RELATED APPLICATION

This is a Divisional of U.S. application Ser. No. 11/971,549 filed Jan.9, 2008 and which claims priority from Japanese Patent Application No.2007-002854. The entire disclosures of the prior applications areincorporated herein in their entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a transflective type LCD device, and moreparticularly, to a transflective type LCD device that has a transmissivearea which transmits light from the rear surface side to the displaysurface side to display an image, and a reflective area which reflectslight incident from the display surface side to display an image.

2. Description of the Related Art

LCD devices are generally classified into transmissive type LCD devicesand reflective type LCD devices. In general, a transmissive type LCDdevice has a backlight source, and controls the transmission amount oflight from the backlight source to thereby display an image. Areflective type LCD device has a reflection film which reflects lightfrom the outside, and utilizes light reflected by the reflection film asa display light source to thereby display an image. The reflective typeLCD device, which does not require a backlight source, is superior inreduction of the power consumption, for a smaller thickness and a lowerweight, as compared with the transmissive type LCD device. However,since the ambient light is used as a display light source, there is adefect that, when the ambient area is dark, the visibility is lowered.

As an LCD device which has the advantage of the transmissive type LCDdevice and that of the reflective type LCD device, there is known atransflective type LCD device (for example, refer to Patent PublicationJP-2003-344837A. The transflective type LCD device has a transmissivearea and a reflective area in each pixel. The transmissive areatransmits light from a backlight source, and sets the backlight sourceas a display light source. The reflective area has a reflection film,and the light incident from the outside and reflected by the reflectionfilm is used as a display light source. In using the transflective typeLCD device, in case the ambient area is bright, the backlight source isturned off, and an image is displayed on the screen by the reflectivearea, which can realize reduction of the power consumption. On the otherhand, in case the ambient area is dark, the backlight source is turnedon, and an image is displayed on the screen by the transmissive area,which can display the image even if the ambient area is dark.

As the display mode of the LCD device, there are an IPS mode(In-plane-Switching mode) and an FFS mode (Fringe-Field-Switching mode)which are the lateral-direction-electric-field mode excellent in thecontrast of transmission and the viewing angle thereof. The LCD deviceof the lateral-direction-electric-field mode such as the IPS mode andFFS mode has a pixel electrode and a common electrode which are formedon the same substrate, and applies an electric field of the lateraldirection to an LC layer. Due to the configuration wherein the LCDdevice of the lateral-direction-electric-field mode displays an image byrotating LC molecules in a direction parallel to the substrate, a higherviewing angle can be realized in the lateral-direction-electric-fieldmode, as compared with an LCD device of the TN mode.

However, in case the transflective type LCD device employs thelateral-direction-electric-field mode such as the IPS mode and FFS mode,as is described in JP-2003-344837A there is raised a problem thatdisplay of dark state (black) and display of bright state (white) areinverted. In the usual drive system, when the transmissive area is setto normally black, the reflective area assumes normally white.Hereinafter, the reason of the inverted display will be described. FIG.20A shows a schematic view indicative of a section of the transflectivetype LCD device, and FIG. 20B shows a schematic view indicative of thepolarized state of light of respective areas when the light advancesfrom a polarizing film, through an LC layer, and to a polarizing film.An arrow represents that the polarized state of light is the linearpolarization, an encircled R represents that the polarized state is theclockwise circular polarization, and an encircled L represents that thepolarized state is the counterclockwise circular polarization. A roundbar represents a director (molecule) of LC.

Each of pixels of an LCD device 50 has a reflective area 55 and atransmissive area 56. The reflective area 55 sets reflected light from areflection film 54 to a display light source, and the transmissive area56 sets a backlight source, not shown, to a display light source. Apolarizing film (first polarizing film) 51 on the viewer side, or frontside, and a polarizing film (second polarizing film) 52 on the rear sideare arranged such that the polarizing axes thereof are perpendicular toeach other. In an LC layer 53, LC molecules are arranged such that thedirection of LC molecules upon absence of applied voltage is deviatedfrom the polarizing axis (light transmission axis) of the secondpolarizing film 52 by 90 degrees. For example, when the polarizing axisof the second polarizing film 52 is at 0 degree, the polarizing axis ofthe first polarizing film 51 is set to 90 degrees, and the longer axisdirection of LC molecules of the LC layer 53 is set to 90 degrees. Inthe LC layer 53, the cell gap is adjusted such that the retardation Δn·d(Δn represents the refractive index anisotropy of LC molecules, and “d”represents the cell gap of LC layer) assumes λ/2 (λ is the wavelength oflight, and, for example, if green light is selected as the standardlight, λ=550 nm) in the transmissive area 56, while the cell gap isadjusted such that the retardation assumes λ/4 in the reflective area55.

Firstly, the operation upon absence of applied voltage on the LC layer53 will be described.

<Reflective Area, Upon Absence of Applied Voltage>

The reflective area upon absence of applied voltage will be described.

In the reflective area 55, linearly polarized light of 90 degreesdirection (longitudinal direction) passing through the first polarizingfilm 51 advances to the LC layer 53. In the LC layer 53, since theoptical axis of the linearly polarized light travelling to the LC layermatches the longer axis direction of LC molecules, the light passesthrough the LC layer 53 with its polarized state being kept at linearlypolarized angle of 90 degrees, and is reflected by the reflection film54. In case of the linearly polarized light, since the light is keptlinearly polarized after being reflected, the light advances to the LClayer 53 again with its polarized state being kept at linearly polarizedangle of 90 degrees. Furthermore, while the light advances from the LClayer 53 to be incident onto the first polarizing film 51 with itspolarized state being kept at linearly polarized angle of 90 degrees,since the polarizing axis of the first polarizing film 51 is also at 90degrees, the light passes through the first polarizing film 51.Accordingly, upon absence of applied voltage, the display represents abright state or black.

<Reflective Area, Upon Presence of Applied Voltage>

The reflective area upon presence of applied voltage will be described.

In the reflective area 55, linearly polarized light of 90 degreesdirection (longitudinal direction) passing through the first polarizingfilm 51 advances to the LC layer 53. Upon presence of applied voltage onthe LC layer 53, the longer axis direction of LC molecules in the LClayer 53 is changed from 0 degree to 45 degrees on the substratesurface. In the LC layer 53, since the polarized direction of theincident light is deviated from the longer axis direction of LCmolecules by 45 degrees, and the retardation of the LC is set to λ/4,linearly polarized light of the longitudinal direction, which advancesto the LC layer 53, advances to the reflection film 54 with itspolarized state being set clockwise-circularly polarized. Thisclockwise-circularly polarized light is reflected by the reflection film54 and has its polarized state being set counterclockwise-circularlypolarized. The counterclockwise-circularly polarized light, whichadvances to the LC layer 53, passes through the LC layer 53 again, andhas its polarized state being set to linearly polarized state of thelateral direction (0 degree direction) to advance to the firstpolarizing film 51. Since the polarizing axis of the first polarizingfilm 51 is at 90 degrees, the light reflected by the reflection film 54cannot be made to pass through, and the display represents a dark state.

As described above, in the reflective area, the display assumes thenormally white display, in which the display represents a bright stateupon absence of applied voltage, while the display represents a darkstate upon presence of applied voltage.

<Transmissive Area, Upon Absence of Applied Voltage>

Next, the transmissive area will be described. Firstly, the state uponabsence of applied voltage will be described.

In the transmissive area 56, linearly polarized light of the lateraldirection passing through the second polarizing film 52 advances to theLC layer 53. In the LC layer 53, since the polarized direction of theincident light is perpendicular to the longer axis direction of LCmolecules, without changing the polarized state, the light passesthrough the LC layer 53 with its polarized state kept linearly polarizedof the lateral direction, and advances to the first polarizing film 51.Since the polarizing axis of the first polarizing film 51 is at 90degrees, the transmitted light cannot pass through the first polarizingfilm 51, and the display represents a dark state.

<Transmissive Area, Upon Presence of Applied Voltage>

Next, the state upon presence of applied voltage will be described. Inthe transmissive area 56, linearly polarized light of the lateraldirection passing through the second polarizing film 52 advances to theLC layer 53. Upon presence of applied voltage on the LC layer 53, thelonger axis direction of LC molecules in the LC layer 53 is changed from0 degree to 45 degrees on the substrate surface. In the LC layer 53,since the polarized direction of the incident light is deviated from thelonger axis direction of LC molecules by 45 degrees, and the retardationof the LC is set to λ/2, linearly polarized light of the lateraldirection, which advances to the LC layer 53, advances to the firstpolarizing film 51 with its polarized state being set to linearlypolarized state of the longitudinal direction. Accordingly, in thetransmissive area 56, the first polarizing film 51 allows the backlightincident onto the second polarizing film 52 to pass therethrough, andthe display represents a bright state or white.

As described above, in the transmissive area, the display assumes anormally black mode, in which the display represents a dark state uponabsence of applied voltage, while the display represents a bright stateupon presence of applied voltage.

As a method to solve above-described problems, JP-2006-180200A describesa device configuration for solving the problem of the display inversionbetween the transmissive area and the reflective area, while using aspecific signal processing and driving technique for the LCD device. TheLCD device described in JP-2006-180200A is a transflective type LCDdevice including a pair of polarizing films which have an LC layersandwiched therebetween. The polarizing films have polarizing axes whichare perpendicular to each other. Each pixel of the LCD device includes atransmissive area and a reflective area and is driven by thelateral-electric-field mode, wherein the longer axis of LC molecules inthe LC layer is parallel or perpendicular to the polarized direction oflight which advances to the LC layer in the transmissive area. Eachpixel has a pixel electrode arranged in a transmissive area and areflective area of the pixel which is driven by a common data signal, afirst common electrode to which a first common signal which is shared byreflective areas of a plurality of pixels is applied, and a secondcommon electrode to which a second common signal which is shared bytransmissive areas of the plurality of pixels is applied.

FIG. 21 shows a schematic view indicative of the planar configuration ina single pixel of the LCD device described in JP-2006-180200A. An LCDdevice 100 includes a first common electrode 137 which corresponds to areflective area 121, a second common electrode 138 which corresponds toa transmissive area 122, and a pixel electrode 135 which supplies acommon data signal to the reflective area 121 and transmissive area 122.In the reflective area 121, the LC layer is driven by an electric fieldgenerated by the pixel electrode 135 and the first common electrode 137,and in the transmissive area 122, the LC layer is driven by the electricfield generated by the pixel electrode 135 and the second commonelectrode 138. In this configuration, since a signal (electricpotential) applied to the first common electrode 137 and a signalapplied to the second common electrode 138 are controlled such that themagnitude of electric field applied to the LC layer in the reflectivearea 121 and the magnitude of electric field applied to the LC layer inthe transmissive area 122 are opposite to each other, the display in thereflective area and the display in the transmissive area have the samedisplay mode. Accordingly, the problem of the transflective type LCDdevice, or the problem of the inversion of display of a bright/darkstate between the reflective area and the transmissive area can besolved.

Specifically, a first common signal and a second common signal suppliedto the first common electrode 137 and the second common electrode 138,respectively, are inverted in synchrony with a pixel signal supplied tothe pixel electrode 135, wherein the first common signal is obtained bysubstantially inverting the second common signal. In this case, forexample, when an electric potential of 5 V is applied to the pixelelectrode 135 in the reflective area 121 and transmissive area 122, bysetting the first common electrode 137 to 0 V, and setting the secondcommon electrode 138 to 5 V, the LC layer can be rotated only in thereflective area 121, and the problem of the inversion of display ofbright state and display of dark state between the reflective area 121and the transmissive area 122 can be solved. In employing thisconfiguration, it is not necessary that the first common signal and thesecond common signal have to be inverted signals in a strict sense. Forexample, the first common signal may assume 0 V or 5 V, and the secondcommon signal may assume 6 V or 0 V. Hereinafter, the drive system forLCD device in JP-2006-180200A is referred to as an inverting drivesystem using an inverting drive scheme, for the sake of convenience.

On the other hand, in the transflective type LCD device, in order toallow the image quality in the reflective mode to match the imagequality in the transmissive mode, it is important that thevoltage-luminance characteristics including VR (voltage-reflectance)characteristics and VT (voltage-transmittance) characteristics in thereflective area matches those in the transmissive area. For example, ina literature entitled “A single Gap Transflective Fringe-Field SwitchingDisplay”, SID2006 P159(p. 810), there is described an LCD device thatperforms the FFS drive mode with the same gap setup in a reflective areaand in a transmissive area. In this technique, the LCD is of thetransflective type and uses the lateral-electric-field mode withoutusing an inverting drive system, wherein an in-cell retarder is usedonly in the reflective area, to optically solve the problem of theinversion between the reflective area and the transmissive area, andthen, the VR characteristics and VT characteristics are allowed to matchbetween the reflective area and the transmissive area. The techniquesolving the problem is such that the transmissive area is driven usingthe FFS-mode drive, and the reflective area is driven using the IPS-modedrive, and the angle formed between electrodes in the form of comb teethand the rubbing angle in the transmissive area is set to approximately80 degrees, and the angle formed between electrodes in the form of combteeth and the rubbing angle in the reflective area is set toapproximately 45 degrees, which makes the VT/VR characteristics matchbetween both the areas. This technique compensates the differencebetween both the drive voltages, which occurs due to the same cell gapprovided in the reflective area and the transmissive area.

In the configuration described in the JP-2006-180200A, both the VTcharacteristics and VR characteristics are opposite to each other. Thatis, the VT characteristics is such that a higher voltage provides ahigher transmittance whereas the VR characteristics is such that ahigher voltage provides a lower reflectance, thereby raising a problemthat the image quality in the reflective mode does not match the imagequality in the transmissive mode. A method to solve the problem of theimage quality in the inverting drive scheme is not known. It is recitedin the above literature that, with respect to the problem that the VRcharacteristic and VT characteristic have a deviation therebetween dueto the same cell gap being provided in the reflective mode andtransmissive mode, only the angle formed between electrodes in the formof comb teeth and the rubbing angle has a difference between thereflective area and the transmissive area. That is, this technique issilent to the solution for allowing the image quality in the reflectivemode to match the image quality in the transmissive mode.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to solve the aboveproblem and to provide a transflective type LCD device driven using theinverting drive scheme, which is capable of allowing the image qualityin the reflective mode to match the image quality in the transmissivemode.

The present invention provides, in a first aspect thereof, atransflective liquid crystal display (LCD) device including an LCD panelhaving an array of pixels each having a reflective area and atransmissive area in a liquid crystal (LC) layer, and a drive circuitfor driving the reflective area and the transmissive area of the LClayer by using an inverting drive scheme, wherein characteristics ofreflectance of the reflective area with respect to a value of [Vr(K)−Vr] and characteristics of transmittance of the transmissive areawith respect to a value of [Vt−Vt (K)] substantially match each other,where Vr and Vt are drive voltages of the LC layer in the reflectivearea and transmissive area, respectively, Vr (K) is a dark-state setupvoltage in the reflective area, and Vt (K) is a dark-state setup voltagein the transmissive area.

The present invention provides, in a second aspect thereof, atransflective liquid crystal display (LCD) device including: an LCDpanel including an array of pixels each having a reflective area and atransmissive area in a liquid crystal (LC) layer; and a drive circuitfor driving the reflective area and the transmissive area of the LClayer by using an inverting drive scheme, wherein: the drive circuitdrives the reflective area and the transmissive area by using drivevoltages Vr and Vt, respectively, the reflective area has a dark-statesetup voltage Vr(K) and a bright-state setup voltage Vr(W), and thetransmissive area having a dark-state setup voltage Vt(K) and abright-state setup voltage VOW); a first characteristic curve forreflectance of the reflective area with respect to a value of [Vr(K)−Vr] and a second characteristic curve for transmittance of thetransmissive area with respect to a value of [Vt−Vt (K)] havetherebetween a relationship such that: a slope of the firstcharacteristic curve in a vicinity of the Vr(K), a slope of the firstcharacteristic curve in a vicinity of the Vr(W), a slope of the secondcharacteristic curve in a vicinity of the Vt(K), and a slope in avicinity of the St(W) substantially match one another.

The above and other objects, features and advantages of the presentinvention will be more apparent from the following description,referring to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view indicative of the configuration of an LCDdevice according to an embodiment of the present invention;

FIG. 2 is a graph indicative of the relationship between an appliedvoltage and the reflectance/transmittance in the LC layer;

FIG. 3 is a graph indicative of the inverted VR characteristics and theVT characteristics in a first example;

FIG. 4 is a graph indicative of another example of the inverted VRcharacteristics and the VT characteristics in the first example;

FIG. 5 is a graph indicative of the inverted VR characteristics and theVT characteristics in a second example;

FIG. 6 is a block diagram indicative of an LCD device including an LCdriver;

FIG. 7 shows a block diagram indicative of the configuration of an LCdriver;

FIG. 8 is a block diagram indicative of another example of theconfiguration of an LCD device including an LC driver in the secondexample;

FIG. 9 is a block diagram indicative of another example of theconfiguration of an LC driver in the second example;

FIG. 10 is a graph indicative of the inverted VR characteristics and theVT characteristics in a third example;

FIG. 11 is a block diagram indicative of the LCD device including an LCdriver in the third example;

FIG. 12 is a block diagram indicative of the configuration of a VCOM-IC;

FIG. 13A and FIG. 13B are timing chart showing waveforms of driving theLCD device to assume a dark state in both the reflective area andtransmissive area;

FIG. 14A and FIG. 14B are timing chart showing waveforms of driving theLCD device to assume a bright state in both the reflective area andtransmissive area;

FIG. 15 is a graph indicative of the inverted VR characteristics and theVT characteristics in a fourth example;

FIG. 16 is a graph indicative of the result of calculating therotational direction of the director;

FIG. 17 is a graph indicative of the V-T curves and the V-R curvescorresponding to respective combinations when the inverting drivingtechnique is used by changing the orientation direction of LC moleculesin the transmissive area and reflective area independently;

FIG. 18 is a top plan view indicative of the state of the electrodearrangement in a pixel of the LCD device in the fourth example;

FIG. 19 is a top plan view indicative of the state of the electrodearrangement in a pixel of the LCD device in a fifth example;

FIG. 20A is a sectional view indicative of the transflective type LCDdevice, and FIG. 20B is a schematic view indicative of the polarizedstate of light in the respective areas when the light is emitted fromthe polarizing film, LC layer, and polarizing film; and

FIG. 21 is a block diagram indicative of the planar configuration in asingle pixel of an LCD device described in JP-2006-180200A.

DETAILED DESCRIPTION OF EMBODIMENTS

Before describing exemplary embodiments of the present invention, theprinciple of the present invention will be described for a betterunderstanding of the present invention.

The present inventors examined necessary conditions to solve the problemof the mismatching in the image quality between the reflective area andthe transmissive area. The result of the experiments revealed thefollowing facts. It is assumed here that the dark-state setup voltageand bright-state setup voltage for LC layer in the reflection area areVr(K) and Vr (W), respectively, the dark-state setup voltage andbright-state setup voltage for LC in the transmissive area are Vt(K) andVt (W), respectively, and the applied voltage on the LC layer in thereflective area and the transmissive are Vr and Vt, respectively. Inthis case, if the relationship between (Vr (K)−Vr) and the reflectancein the reflection area, which relationship is referred to as inverted VRcharacteristic and represented by ([Vr (K)−Vr]−R) characteristics, isallowed to match the relationship between (Vt−Vt (K)) and thetransmittance in the transmission area, which relationship is referredto as VT characteristic and represented by ([Vt−Vt (K)]−T)characteristics, this is equivalent to match the image quality in thereflection area and the image quality in the transmission area.

Now, exemplary embodiments of the present invention will be describedwith reference to accompanying drawings, wherein similar constituentelements are designated by similar reference numerals throughout thedrawings.

FIG. 1 shows the configuration of an LCD device according to anexemplary embodiment of the present invention. The LCD device 10includes an LCD panel that includes a pair of transparent substrates,which include counter substrate 12 and TFT substrate 14, an LC layer 13sandwiched between the paired transparent substrates 12, 14, and a pairof polarizing films 11, 15, which are provided on the sides of thepaired transparent substrates far from the LC layer 13 and arranged suchthat the polarizing axes thereof extend perpendicular to each other. TheLCD device 10 further includes a backlight source or backlight unit, notshown, arranged on the surface of the LCD panel far from the viewer ofthe LCD device. In the LC layer 13, LC molecules are so arranged as tobe substantially parallel to the transparent substrates, and the LCDdevice 10 is configured as an LCD device of the lateral-electric-fieldmode (IPS mode).

The LCD panel has a reflective area 21 and a transmissive area 22. Inthe transmissive area 22, there are formed a transmissive-area pixelelectrode 36 and a transmissive-area common electrode 38, to generate anelectric field in a direction substantially parallel to the transparentsubstrates. In the reflective area 21, there is arranged a reflectionfilm 16 that reflects light incident from the side of the polarizingfilm 11, and allows the reflected light to pass by the polarizing film11. On the reflection film 16, a transparent insulating film 17 isformed, and on the transparent insulating film 17, a reflective-areapixel electrode 35 and a reflective-area common electrode 37 areprovided to generate an electric field therebetween in a directionsubstantially parallel to the substrates.

The reflection film 16 is configured as a micromirror so as to scatterthe incident light in a variety of directions. The micromirror isconfigured by forming concavities and convexities on a photosensitiveresin by employing the photolithographic and stamping technique, andarranging a metal film made of Al, Ag or an alloy thereof on the thusformed concavities and convexities. With respect to the thickness of theLC layer 13, in the transmissive area 22, the cell gap is formed suchthat the phase difference of the LC layer 13 assumes ½ upon presence ofapplied voltage, and, in the reflective area 21, the cell gap is formedsuch that the phase difference of the LC layer 13 assumes ¼ uponpresence of applied voltage. That is, the cell gap dr in the reflectivearea 21 is approximately half the cell gap dt in the transmissive area22.

Hereinafter, an example of the present embodiment will be described. Ina first example, the clearance of the comb teeth electrodes, which isdefined as clearance Lr between the reflective-area common electrode 37and the reflective-area pixel electrode 35, is suitably determined inthe reflective area 21. The inverted VR characteristics and the VTcharacteristics match each other. As the LC layer 13, a layer having arefractive index anisotropy set to Δn=0.090, and a permittivityanisotropy set to Δ ε=13.5 is used. With respect to the transmissivearea 22, so as to set the drive voltage to 5 V or lower, which istypical as the output voltage of an LC driver, the cell gap is set todt=3.5 μm, the comb teeth width, which is the width of thetransmissive-area pixel electrode 36 and transmissive-area commonelectrode 38, is set to wt=3 μm, and the comb teeth clearance, which isthe clearance between the reflective-area common electrode 37 and thereflective-area pixel electrode 35 is set to Lr=9 μm.

In general, in a lateral-electric-field mode LCD device, assuming thatthe cell gap is set to “d”, and the comb teeth clearance is set to “1”,the threshold voltage Vth, which is an initial rise voltage of thereflectance in the reflectance-voltage characteristics, is proportionalto (1/d). Thus, with respect to the reflective area 21, considering thecell gap is dr=1.8 μm, the comb teeth width is selected at wr=3 μm, andthe comb teeth clearance is selected at lr=4.5 μm. The relationshipbetween the applied voltage and the reflectance/transmittance of the LClayer in this case is shown in FIG. 2. In this figure, the dark-statesetup voltage Vt (K) and bright-state setup voltage Vt (W) in thetransmissive area 22 are set to Vt (K)=0 V and Vt (W)=5 V, thedark-state setup voltage Vr (K) and bright-state setup voltage Vr (W) inthe reflective area 21 are set to Vr (K)=5 V and Vr (W)=0 V, and theinverted VR characteristics and the VT characteristics are plotted. Itwill be understood from FIG. 3 that the inverted VR characteristics andthe VT characteristics approximately match each other.

In FIG. 3, although it can be seen that the inverted VR characteristicsand the VT characteristics approximately match each other, if the imagequality in the transmissive mode is made optimum, the image quality inthe reflective mode is deviated toward a bright state. Accordingly, soas to realize the further optimization, in the reflective area 21, thecomb teeth width wr is set to 3 μm, and the comb teeth clearance lr isset to 3.0 μm. The inverted VR characteristics and the VTcharacteristics in this case are shown in FIG. 4. It will be understoodfrom FIG. 4 that the inverted VR characteristics and the VTcharacteristics further match each other as compared with the case shownin FIG. 3, and it can be seen that the image quality in the reflectivemode and the image quality in the transmissive mode further match eachother. On the other hand, in this case, since the LC molecules on thecomb teeth do not rotate, and only the LC molecules on the gap betweenthe comb teeth rotate, whereby the contrast ratio in the reflective modeis lowered.

Next, the second example will be described. In this example, byimproving the manner of driving the LC layer 13, the inverted VRcharacteristics and the VT characteristics are allowed to further matcheach other without lowering the contrast ratio in the reflective mode.As the LC layer 13, similar to the first example, a layer having arefractive index anisotropy set to Δn=0.090, and a permittivityanisotropy set to Δε=13.5 is used. Furthermore, in the transmissive area22, the cell gap dt is set to 3.5 μm, the comb teeth width wt is set to3 μm, and the comb teeth clearance lt is set to 9 μm. With respect tothe comb teeth width wr and comb teeth clearance lr in the reflectivearea 21, considering that the threshold voltage of LC is proportional to(1/d), and that the cell gap is dr=1.8 μm, it is determined that wr=3μm, and lr=4.5 μm.

The relationship between the applied voltage and thereflectance/transmittance of the LC layer in this case is shown in FIG.2. In FIG. 2, the dark-state setup voltage Vt (K) and bright-state setupvoltage Vt (W) in the transmissive area 22 are set to Vt (K)=0 V and Vt(W)=5 V, and the dark-state setup voltage Vr (K) and bright-state setupvoltage Vr (W) in the reflective area 21 are set to Vr (K)=5 V and Vr(W)=0 V, and the inverted VR characteristics and the VT characteristicsare plotted, whereby a graph shown in FIG. 3 is obtained. It will beunderstood from FIG. 3 that the inverted VR characteristics and the VTcharacteristics approximately match each other. However, it can be seenthat, when the image quality in the transmissive mode is made optimum,the image quality in the reflective mode is deviated toward the brightstate.

Accordingly, the LC drive voltage is optimized so that the inverted VRcharacteristics and the VT characteristics match each other. FIG. 5shows the inverted VR characteristics and the VT characteristics whenthe dark-state setup voltage Vr (K) and bright-state setup voltage Vr(W) in the reflective area 21 are set to Vr (K)=6.5 V and Vr (W)=0 V,and the dark-state setup voltage Vt (K) and bright-state setup voltageVt (W) in the transmissive area 22 are set to Vt (K)=0 V and Vt (W)=5 V.It will be understood from FIG. 5 that, by setting the applied voltagefor the dark-state setup in the reflective area 21 different from theapplied voltage for the bright-state setup in the transmissive area 22,it is possible to allow the inverted VR characteristics and the VTcharacteristics match each other, which can allow the image quality inthe reflective mode and the image quality in the transmissive mode matcheach other.

Next, a specific example of the method of generating data signals duringthe line-selection time period will be described. The generated datasignals include a data signal (reflective electric potential)corresponding to the reflective area 21 in the reflective-area selectiontime period, and a data signal (transmissive electric potential)corresponding to the transmissive area 22 in the transmissive-areaselection time period. FIG. 6 shows an LCD device including an LCD panel20 and an LC driver 40 that drives the LCD panel 20. To the LC driver40, a timing signal used for timing of signal transmission, and digitalsignals (D (n, m)) of, for example, approximately RGB 8 bitscorresponding to respective pixels are input in series for therespective pixels. Based on the input pixel signals and timing signal,the LC driver 40 generates a gate signal to be supplied to a gate linecorresponding to the reflective area 21 and a gate line corresponding tothe transmissive area 22, a data signal to be supplied to a data line32, and a common electrode signal to be supplied to a common electrodeline 39. The common electrode line 39 is connected to thereflective-area common electrode 37 in the reflective area 21 and to thetransmissive-area common electrode 38 in the transmissive area 22.

FIG. 7 shows the configuration of the LC driver 40. The LC driver 40includes a timing controller 41, a line memory 42, a LUT (look-up table)circuit 43, a selection circuit (MUX circuit) 44, a digital-to-analogconversion (DAC) circuit 45, a voltage generation circuit 46, and a COMsignal circuit 47. The timing controller 41 includes a gate-timinggeneration section and a data-timing generation section, and generates avariety of timing signals based on the input timing signal. The LCdriver 40 separates the timing for one gate line into a first timing forthe reflective area (reflective-area selection time period) and a secondtiming for the transmissive area (transmissive-area selection timeperiod), and drives the gate lines in the reflective area andtransmissive area under the separate timings. Respective gate signalsare generated in the LC driver 40, and are supplied to the gate linecorresponding to the reflective area 21 and the gate line correspondingto the transmissive area 22. In an alternative, gate signals may begenerated using a shift register configured by TFTs on the TFTsubstrate.

The line memory 42 stores therein input digital pixel signals D (n, m)for one data line. The LUT circuit 43 performs the gradation conversionin accordance with an LUT for a pixel gradation conversion means in thetransmissive area. The MUX circuit 44 selectively outputs a digitalpixel signal to be stored in the line memory 42, and a digital pixelsignal which has its gradation converted by the LUT circuit 43. The DACcircuit 45 generates, based on the digital pixel signal input from theMUX circuit 44, and the voltage generated by the voltage generationcircuit 46, a voltage signal (data signal) corresponding to thegradation for the digital pixel signal. The COM signal circuit 47generates a common electrode signal to be supplied to the commonelectrode line 39 of the respective pixels.

The digital pixel signals D (n, m) input to the LC driver 40 aretemporarily stored in the line memory 42. The LUT circuit 43 performsthe gradation conversion in accordance with the LUT, and generates adigital pixel signal for the transmissive area corresponding to thetransmissive mode. The MUX circuit 44 selects, in the transmissive-areaselection time period, the digital pixel signal for the transmissivearea which is generated by the LUT circuit 43, and delivers the thusselected digital pixel signal for the transmissive area to the DACcircuit 45. Furthermore, the MUX circuit 44 selects, in thereflective-area selection time period, a digital pixel signal for thereflective area which is stored in the line memory 42 and does not passthrough the LUT circuit 43, and delivers the thus selected digital pixelsignal for the reflective area to the DAC circuit 45. Accordingly,digital pixel signals which are different in gradation are input to theDAC circuit 45 under the reflective mode and transmissive mode.

In this case, a reference voltage VDD upon forming a signal voltage inthe LC driver 40 is set to VDD=6.5 V, and in the voltage generationcircuit 46, reference voltages of V=0 V, . . . =6.5 V are generated. Asthe LC driver, a driver of 8 bits (256 gradations) is used, and avoltage is arbitrarily selected therefrom, and 64 gradations and 6 bitsare displayed. The DAC circuit 45 outputs voltages corresponding toinput 0 to 255 gradations, in the reflective-area selection time period,and outputs 6.5 V for 0 gradation, 5 V for 5 gradation, and 0 V for 255gradation. That is, the DAC circuit 45 outputs, in the reflective mode,a signal of 6.5 V to the data line 32 corresponding to 0 gradation (darkor black) of a digital pixel signal, and, a signal of 0 V to the dataline 32 corresponding to 255 gradation (bright or white).

The LUT circuit 43 outputs, using a LUT, 255 to 5 gradation with respectto 0 gradation to 255 gradation of the input digital pixel signal. Thatis, when an input digital pixel signal is 0 gradation (black), the LUTcircuit 43 outputs 255 gradation to the DAC circuit 45, and when aninput digital pixel signal is 255 gradation (white), the LUT circuit 43outputs 5 gradation to the DAC circuit 45. Accordingly, the DAC circuit45 outputs, in the transmissive mode time (transmissive-area selectiontime period), corresponding to 0 gradation (black) of an input digitalsignal, a signal of 0 V being the voltage for 255 gradation in thereflective mode to the data line 32, and outputs, corresponding to 255gradation (white), a signal of 5 V being the voltage for 5 gradation inthe reflective mode to the data line 32. By performing the gradationconversion in the LUT circuit 43, it is possible to realize dark-statesetup voltage Vr (K)=6.5 V and bright-state setup voltage Vr (W)=0 V inthe reflective area 21, and dark-state setup voltage Vt (K)=0 V andbright-state setup voltage Vt (W)=5 V in the transmissive area 22.

In the above description, the digital pixel signal for the transmissivearea is generated using the line memory 42, and the signal voltagesupplied to the data line 32 in the reflective-area selection timeperiod is different from that in the transmissive-area selection timeperiod. On the other hand, instead of this configuration, there may beemployed a configuration in which a digital pixel signal for thetransmissive area is generated from each input digital pixel signal.More specifically, as shown in FIG. 8, a data line is separated into areflective data line 32 a and a transmissive data line 32 b. An LCdriver 40 a for driving the data lines is shown in FIG. 9, wherein aninput digital pixel signal is input to the LUT circuit 43 without usinga line memory. The digital pixel signal for the transmissive area isgenerated from the input digital pixel signal corresponding to thegradation in the reflective area 21 by using the LUT circuit 43.Accordingly, an operation similar to the above-described operation canbe realized.

As described above, different amplitudes are provided to the commonsignal for the reflective area and the common signal for thetransmissive area in the present embodiment. This allows the drivevoltage for the reflective area to be larger than the drive voltage forthe transmissive area. It is noted in the first embodiment that, if thebright-state setup voltage Vt(W) in the transmissive area is to be equalto the dark-state setup voltage Vr(K) in the reflective area, aclearance lr of 3 μm or smaller should be assured between the electrodesin the reflective area for the case of a clearance of lt=9 μm beingprovided between the electrodes in the transmissive area. On the otherhand, in the present embodiment, it is sufficient that the clearance ltbetween the electrodes in the reflective area be as large as 4.5 μm.Accordingly, the area of the comb-teeth electrodes per pixel area may berelatively smaller in the present embodiment, whereby a larger area canbe assured for the gap between the comb-teeth electrodes, where the LCmolecules are driven by the drive voltage. This solves the problem of apossible reduction of the contrast ratio in the reflective mode, whichmay arise in the first embodiment.

Next, a third example will be described. In the third example, similarto the second example, by improving the manner of driving the LC layer13, the inverted VR characteristics and the VT characteristics are madeto match each other. As the LC layer 13, similar to the first and secondexamples, a layer having a refractive index anisotropy of Δn=0.090, anda permittivity anisotropy of Δε=13.5 is used. Furthermore, in thetransmissive area 22, the cell gap dt is set to 3.5 μm, the comb teethwidth wt is set to 3 μm, and the comb teeth clearance lt is set to 9 μm.With respect to the comb teeth width wr and comb teeth clearance lr inthe reflective area 21, considering that the threshold voltage of LC isproportional to (1/d) and that the cell gap is dr=1.8 μm, it isdetermined that wr=3 μm, and lr=4.5 μm.

FIG. 10 shows the inverted VR characteristics and the VT characteristicswhen the dark-state setup voltage Vr (K) and bright-state setup voltageVr (W) in the reflective area 21 are set to Vr (K)=6.0 V and Vr (W)=1.0V, and the dark-state setup voltage Vt (K) and bright-state setupvoltage Vt (W) in the transmissive area 22 are set to Vt (K)=0 V and Vt(W)=5 V. In FIG. 10, by setting the applied voltage for the dark-statesetup in the reflective area 21 different from the applied voltage forthe bright-state setup in the transmissive area 22, it is possible toallow the inverted VR characteristics and the VT characteristics tomatch each other. This configuration allows the image quality in thereflective mode and the image quality in the transmissive mode to matcheach other.

The difference between this example and the second example is that, inthe second example, the voltage supplied to the data line in thereflective-area selection time period is different from that in thetransmissive-area selection time period to realize the inverting drivescheme, whereas in this example, the voltage supplied to the commonelectrode in the reflective area 21 is different from that in thetransmissive area 22 to realize the inverting drive scheme. Thisinverting drive scheme can be realized in the following manner. FIG. 11shows an LCD device of the third example, which includes an LCD paneland an LC driver. In respective pixels, corresponding to the reflectivearea 21 and transmissive area 22, a TFT-R 33 and a TFT-T 34 are arrangedas switching elements. Furthermore, in the display area, a common gateline 31 for driving the TFT-R 33 and TFT-T 34, and a common data line 32that supplies a pixel signal to the pixel electrode through the TFT areso formed as to be perpendicular to each other.

In each pixel, the reflective-area pixel electrode 35 (FIG. 1) andtransmissive-area pixel electrode 36 are formed in the reflective area21 and transmissive area 22, respectively. The reflective-area pixelelectrode 35 and transmissive-area pixel electrode 36 each have aportion extending in parallel to the gate line 31 and another portionprotruding in the display area. In the reflective area 21, thereflective-area common electrode 37 is formed at a position opposing thereflective-area pixel electrode 35 on the plane of the substratesurface. In the transmissive area 22, the transmissive-area commonelectrode 38 is formed at a position opposing the transmissive-areapixel electrode 36 on the plane of the substrate surface. Thereflective-area common electrode 37 and transmissive-area commonelectrode 38 are supplied with predetermined signals which are shared byrespective pixels in the LCD device (reflective-area common electrodesignal and transmissive-area common electrode signal).

To an LC driver 40 b, a timing signal for LC, and digital signals of,for example, approximately RGB 8 bits corresponding to respective pixelsare input in series for the respective pixels. The LC driver 40 bgenerates, based on the input pixel signals and timing signal, a gatesignal to be supplied to the gate line 31, and a data signal to besupplied to the data line 32. Furthermore, the LC driver 40 b generatesa transmissive-area common electrode signal T-COM to be supplied to thetransmissive-area common electrode 38 disposed in the transmissive area22. The transmissive-area common electrode signal T-COM output from theLC driver 40 b is input to a VCOM-IC 48. The VCOM-IC 48 inverts thetransmissive-area common electrode signal T-COM, and generates areflective-area common electrode signal R-COM which has its amplitudeamplified.

The configuration of the VCOM-IC 48 is shown in FIG. 12. A DC-DCconverter 401 and a regulator 402 are configured as a voltage step-upcircuit that generates a voltage Vcom for the common electrode signalfrom a logic voltage VCC. An inverting amplifier 403 inverts thetransmissive-area common electrode signal T-COM. The signal inverted bythe inverting amplifier 403 is output as the reflective-area commonelectrode signal R-COM through an R-C circuit 404 that adjusts thecenter voltage. The center voltage of the reflective-area commonelectrode signal R-COM is set to a voltage which is equal to the medianof the amplitude of a pixel electrode signal and the transmissive-areacommon electrode signal.

The operation in this example will be described. In this operation, areference voltage VDD for generating a signal voltage in the LC driver40 b is selected at VDD=5 V. The amplitude of the pixel electrode signaland the transmissive-area common electrode signal T-COM is set to 0 Vthrough 5 V. The VCOM-IC 48 can generate a voltage (Vcom) of 7 V, andthe amplitude of the reflective-area common electrode signal R-COMgenerated by the VCOM-IC 48 is set to 0 V through 7 V.

Firstly, display of dark state will be described. FIG. 13A and FIG. 13Bshow drive waveforms of display for dark state in the reflective area 21and transmissive area 22, respectively. In the transmissive area 22, asshown in FIG. 13B, the phase of a pixel voltage (Pixel) to be suppliedto the transmissive-area pixel electrode 36 and the phase of the voltageof the transmissive-area common electrode signal T-COM to be supplied tothe transmissive-area common electrode 38 match each other, and theamplitude is 5 V, respectively. Accordingly, the drive voltage assumes 0V, and dark-state setup voltage Vt (K)=0 V. On the other hand, in thereflective area 21, as shown in FIG. 13A, the amplitude of a pixelvoltage (Pixel) to be supplied to the reflective-area pixel electrode 35is 5 V, which is equal to that in the transmissive area 22, and thephase of the voltage of the reflective-area common electrode signalR-COM to be supplied to the reflective-area common electrode is oppositeto the phase of the T-COM, and the amplitude is 7 V. Accordingly, anoffset voltage is provided in the reflection, and the drive voltage inthe reflective area 21 is set to (5 V+7 V)/2=6.0 V, and the dark-statesetup voltage Vr (K) assumes 6 V.

Next, display in white will be described. FIG. 14A and FIG. 14B showdrive waveforms of display in white in the reflective area 21 andtransmissive area 22, respectively. In the transmissive area 22, asshown in FIG. 14B, the phase of a pixel voltage (Pixel) to be suppliedto the transmissive-area pixel electrode 36 and the phase of the voltageof the transmissive-area common electrode signal T-COM to be supplied tothe transmissive-area common electrode 38 are opposite to each other,and the amplitude is 5 V, respectively. Accordingly, the drive voltageassumes 5 V, and bright-state setup voltage Vt (W)=5 V. On the otherhand, in the reflective area 21, as shown in FIG. 14A, the amplitude ofa pixel voltage (Pixel) to be supplied to the reflective-area pixelelectrode 35 is 5 V, which is equal as in the transmissive area 22, andthe phase of the voltage of the reflective-area common electrode signalR-COM to be supplied to the reflective-area common electrode match thephase of the T-COM, and the amplitude is 7 V. Accordingly, an offsetvoltage is provided in the reflection, and the drive voltage in thereflective area 21 is set to (7 V+5 V)/2=1.0 V, and the bright-statesetup voltage Vr (W) assumes 6V.

In the above-described operation, the dark-state setup voltage Vr (K)and bright-state setup voltage Vr (W) in the reflective area 21 are setto Vr (K)=6.0 V and Vr (W)=1.0 V, respectively, and the dark-state setupvoltage Vt (K) and bright-state setup voltage Vt (W) in the transmissivearea 22 can be set to Vt (K)=0 V and Vt (W)=5 V, respectively.Accordingly, it is possible to allow the inverted VR characteristics andthe VT characteristics to match each other. This allows the imagequality in the reflective area 21 and the image quality in thetransmissive area 22 to match each other.

Next, a fourth example will be described. In the fourth example, bysuitably setting up the rubbing angle of an orientation film for the LClayer, the inverted VR characteristics and the VT characteristics areallowed to match each other. Before describing the details of thisexample, in a transflective type LCD device which is driven by theinverting drive scheme, the investigation result of how the VTcharacteristics in the transmissive area 22 (FIG. 1) and the VRcharacteristics in the reflective area 21 are allowed to match eachother will be described.

It is assumed here that the voltages applied to the reflective area 21and transmissive area 22 are Vr and Vt, respectively, the black voltageand white voltage in the reflective area 21 are Vr (K) and Vr (W),respectively, the black voltage and white voltage in the transmissivearea 22 are o Vt (K) and Vt (W), respectively, the reflectance is R, andthe transmittance is T. It is also assumed that the slopes of thereflectance R in the vicinity of Vr (K), Vr (W) in the Vr−Rcharacteristics (VR characteristics) are Sr (K), Sr (W) respectively.Similarly, slopes of the transmittance T in the vicinity of Vt (K), Vt(W) in the Vt-T characteristics (VT characteristics) are set to St (K),St (W). At this stage, [Vr (K)−Vr]−R characteristics and [Vt−Vt (K)]−Tcharacteristics are considered.

FIG. 15 shows the VR characteristics and VT characteristics when thedrive voltage is changed. With respect to these characteristics, when itis determined that Vt (K)=0 V, Vr (K)=6 V, and [6−Vr]−R characteristicsand [Vt−0]−T characteristics are plotted, a graph shown in FIG. 3 isobtained. Referring back to FIG. 3, it can be seen that, especially inthe vicinity of black where the reflectance and transmittance is 0, andin the vicinity of white where the reflectance and transmittance is 1,with respect to the slopes of the reflectance characteristics Sr (K), Sr(W), the characteristics are dislocated by the slopes of thetransmittance characteristics St (K), St (W).

That is, in the transmissive area, in the range from absence of appliedvoltage to a threshold voltage Vt−sh, since LC molecules scarcely move,the display is not changed to stay black, and the slope St (K) of thetransmittance in the vicinity of the black voltage Vt (K) is moderate,approaching to display of white. That is, the slope St (W) of thetransmittance at the end of the LC rotation where the orientation of LCmolecules assumes approximately 45 degrees is steep. On the other hand,in the reflective area, in the range from absence of applied voltage toa threshold voltage Vr−sh, since LC molecules scarcely move, the displayis not changed to stay white, and the slope Sr (W) of the reflectance inthe vicinity of the white voltage Vr (W) is moderate, approaching todisplay of black. That is, the slope Sr (K) of the reflectance at theend of the LC motion where the orientation of LC molecules assumesapproximately 45 degrees is steep.

From the above description, the slope Sr (K) of the reflectance and theslope St (K) of the transmittance upon display of black, and the slopeSr (W) of the reflectance and the slope St (W) of the transmittance upondisplay of white do not match each other, and the gradation is to bedislocated by that amount. Accordingly, it was found that allowing theslope Sr (K) of the reflectance and the slope St (K) of thetransmittance upon display of black, and the slope Sr (W) of thereflectance and the slope St (W) of the transmittance upon display ofwhite match each other will lead to the matching of both thecharacteristics.

Investigation was performed with respect to the change in orientation ofLC when an electric field is applied to LC molecules. In general, withthe case θ being an angle between the orientation direction of LCmolecules and the direction of an electric field, the following torque:Torque=(εE×n)×n=ε ₀ε_(a) E sin(θ)cos(θ)will be applied to LC molecules, where EA is permittivity anisotropy.The direction of the electric field is perpendicular to the comb teethelectrodes, and, when the rubbing angle formed by the rubbing directionand the comb teeth electrodes is set to θ=90 degrees−α.

Calculating the above-described mathematical expression for the torque,the torque is maximum when θ=45 degrees. Accordingly, considering a casein which the orientation of LC molecules is rotated by an electric fieldby 45 degrees from the initial orientation, when the angle θ formed bythe orientation direction of LC molecules in the reflective area and thedirection of the electric field is set close to 45 degrees, thethreshold voltage will be small. On the other hand, when the angle θ isset close to 90 degrees, conversely, the threshold voltage will belarge. When the angle θ formed between the orientation direction of LCmolecules and the direction of the electric field is close to 90degrees, since the initial torque is small, the threshold voltage ishigh, and, increases when the orientation of LC molecules is rotatedtoward close to 45 degrees. However, when the angle θ formed between theorientation direction of LC molecules and the direction of the electricfield is set close to 45 degrees, the torque decreases while thethreshold voltage is lowered. Therefore, LC molecules become hard torotate even if the electric field is applied, and the drive voltage willbe increased.

Actually, by paying notice to the manner of the orientation of LCmolecules when an electric field is applied to the LC molecules, therotation direction of a director upon presence of applied voltage iscalculated with the angle θ formed between the orientation direction ofLC molecules and the direction of the electric field being set from 55degrees to 85 degrees at every 10 degrees. The result of the calculationis shown in FIG. 16. When the angle θ formed between the orientationdirection of LC molecules and the direction of then electric field is at90 degrees, since the rotation direction of LC molecules is notdetermined with respect to an electric field, the calculation is notperformed. Furthermore, when the angle θ formed between the orientationdirection of LC molecules and the direction of the electric field is 45degrees or more, due to the need of rotation by 45 degrees from theinitial orientation direction, the orientation is not performed for adirection perpendicular with respect to the comb teeth electrodes ormore. Therefore, the calculation is not performed.

As shown in FIG. 16, when the angle θ formed between the orientationdirection of LC molecules and the direction of the electric field is 85degrees, the LC molecules scarcely rotate before the applied voltageassumes 2 V, and rotate gradually from 2 V, and steeply and largelyrotate in the vicinity of 4 V and form an angle of 45 degrees withrespect to the initial orientation at 6V. On the other hand, as theangle assumes 75 degrees, 65 degrees, 55 degrees, while LC moleculesstart to rotate under a smaller voltage, the torque becomes maximum whenthe angle θ formed between the orientation direction of LC molecules andthe direction of an electric field is 45 degrees. In an area where theangle is larger, that is, in an area where the angle surpasses 30degree, 20 degree, 10 degree, conversely, the change amount of rotationbecomes small when the angle θ formed between the orientation directionof LC molecules and the direction of the electric field is small.Accordingly, it can be seen that, from the rubbing angle, the voltagenecessary when LC molecules are oriented to form 45 degrees by anelectric field becomes large as 6.2 V, 7 V, and 9V.

From the above-described results, it is considered that, when settingthe angle θ formed between the orientation direction of LC and thedirection of an electric field to a smaller direction from 75 degrees,since the threshold voltage under which the LC molecules rotates becomessmall, and the difference of the change amount between a case in whichthe manner of rotation of LC molecules changes from 45 degrees to 22.5degrees direction and a case in which the manner changes from 0 degreeto 22.5 degrees direction becomes small, the V-T characteristics and theV-R characteristics when the inverting drive scheme is performed matcheach other. Actually, an LC panel was formed such that the angle θformed between the orientation direction of LC and the direction of anelectric field assumed 85 degrees, 75 degrees, 65 degrees, and thechange of the transmittance and the reflectance with respect to thevoltage was evaluated.

The V-T curve and the V-R curve are compared against each other whilethe inverting drive scheme is performed by changing the orientationdirection of LC molecules in the transmissive area and reflective areaindependently of each other to 65 degrees, 75 degrees, 85 degrees,respectively. In FIG. 17, V-T curves and V-R curves corresponding torespective combinations are shown. With respect to combinations in whichthe orientation direction of LC molecules in the transmissive area isdifferent from that in the reflective area, it can be realized bychanging the electrode direction in the transmissive area and theelectrode direction in the reflective area. Otherwise, it can berealized by, with the electrode direction being fixed, performing therubbing using masking, or changing only the orientation direction byirradiating an ion beam.

In FIG. 17, with respect to any one of the transmissive area andreflective area, or both of them, in case of setting the orientationdirection such that the rubbing angle is equal to 85 degrees or more,the V-T characteristics in the transmissive area and the V-Rcharacteristics in the inverted reflective area (that is, [Vr (K)−V]−Rcharacteristics) do not match each other, and the visibility is notdesirable. On the other hand, in case the orientation direction in thetransmissive area is set to 75 degrees, and the orientation direction inthe reflective area is set to 65 degrees, the V-T curve in thetransmissive area and the V-R curve in the inverted reflective areaoverlap each other, to provide a desirable visibility. Furthermore, incase the orientation direction is set to 65 degrees in both thetransmissive area and reflective area, the V-T curve in the transmissivearea and the V-R curve in the inverted reflective area can be made tomatch each other completely.

In case the width between the reflective-area common electrode and thereflective-area pixel electrode is Lr, and the width between thetransmissive-area common electrode and the transmissive-area pixelelectrode is Lt, since the cell gap in the transmissive area correspondsto λ/2, and the cell gap in the reflective area corresponds to λ/4. Whenit is determined that Lt=Lr, as shown in FIG. 2, the black voltage Vr(K) in the reflective area is large as compared with the white voltageVr (W) in the transmissive area. Accordingly, so as to allow the drivevoltages to match each other, it is necessary to set up Lr<Lt. In FIG. 3and FIG. 17, the LCD device is formed such that Lr=4 μm and Lt=9 μm.

Under the above-described configuration, the transflective type LCDdevice was manufactured as the fourth example. The configuration of thusformed LCD device was similar to the configuration shown in FIG. 1. A LCmaterial having a refractive index anisotropy of Δn=0.090 was used. Withrespect to the cell gap, the cell gap in the reflective area was 2 μm,and the cell gap in the transmissive area was 3 μm. The width betweenelectrodes in the reflective area and transmissive area was Lr=4 μm andLt=9μm so as to set the drive voltages equal with each other.

FIG. 18 shows the configuration of the electrode arrangement in a pixel.The angle formed between the electrode direction of the comb teethelectrodes including the pixel electrode 35 and common electrode 37 inthe reflective area 21 and the rubbing direction of the orientation filmfor the LC was set to α(R), and the angle formed between the electrodedirection of the comb teeth electrodes including the pixel electrode 36and common electrode 38 in the transmissive area 22 and the rubbingdirection of the orientation film for the LC was set to a (T).Considering the angle formed between the rubbing angle and the combteeth electrodes, α(R) and α(T) are set to a (R)=α(T)=25 degrees. Sincethe threshold voltage in the transmissive area and the threshold voltagein the reflective area can be made small by this configuration, thegradation luminance characteristics in both the areas matched eachother.

The fifth example will be described. In the fourth example, gradationluminance characteristics match each other due to the setting ofα(R)=α(T)=25 degrees. On the other hand, in the fourth example, whenpaying notice to the display in the transmissive area, the transmittanceis minimum upon absence of applied voltage. However, even if an appliedvoltage is small, LC molecules will eventually rotate due to an electricfield, and there may be an increased risk of leakage light. Accordingly,there is raised a problem that the black luminance increases due to thedispersion of the initial orientation of LC molecules and the dispersionof the applied voltage, which fact lowers the contrast ratio. In thisexample, substantially without lowering the gradation luminancecharacteristics in the transmissive area and reflective area, thelowering of contrast ratio in the transmissive area is suppressed.

In the fifth example, considering the angle formed between the rubbingangle of the orientation film and the comb teeth electrodes, which hasbeen described in the fourth example, α(R) and α(T) are set to α(R)=25degrees and α(T)=15 degrees. In this way, the gradation luminancecharacteristics match each other in the transmissive area and reflectivearea, and the lowering of contrast ratio in the transmissive area can besuppressed.

FIG. 19 shows the configuration of the electrode arrangement in a pixelin the fifth example. In this example, the electrode direction of thecomb teeth electrodes including pixel electrode 35 and common electrode37 in the reflective area 21 is different from the electrode directionof the comb teeth electrodes including pixel electrode 36 and commonelectrode 38 in the transmissive area 22, and the rubbing direction ofthe orientation film in the reflective area 21 is equal to the rubbingdirection of the orientation film in the transmissive area 22. Thisallows the angle formed between the rubbing direction and the electrodedirection of the comb teeth electrodes in the transmissive areadifferent from that in the reflective area.

Instead of the above described configuration, there may be employed aconfiguration in which, as shown in FIG. 18, by allowing the electrodedirection in the reflective area 21 equal to the electrode direction inthe transmissive area 22, and allowing the rubbing direction in thereflective area 21 to be different from the rubbing direction in thetransmissive area 22, the angle formed between the rubbing direction andthe electrode direction of the comb teeth electrodes in the transmissivearea is different from that in the reflective area. In this case, bymasking only the transmissive area 22 and performing the rubbingprocessing for only the reflective area 21, and then masking only thereflective area 21 and performing the rubbing processing for only thetransmissive area 22, different rubbing directions are realized in thereflective area 21 and transmissive area 22 of the orientation film. Theorientation processing is not restricted to the rubbing processing, andthere may be employed a configuration in which the orientationrestraining force of LC molecules is achieved using light and ion beam.

While the invention has been particularly shown and described withreference to exemplary embodiment and modifications thereof, theinvention is not limited to these embodiment and modifications. It willbe understood by those of ordinary skill in the art that various changesin form and details may be made therein without departing from thespirit and scope of the present invention as defined in the claims.

What is claimed is:
 1. A transflective liquid crystal display (LCD)device comprising an LCD panel including an array of pixels each havinga reflective area and a transmissive area in a liquid crystal (LC)layer, and a drive circuit for driving said reflective area and saidtransmissive area of said LC layer by using an inverting drive scheme,wherein the drive circuit generates a first common electrode signalperiodically inverted in a polarity thereof at a predeterminedamplitude, and generates a second common electrode signal having a phaseopposite to a phase of the first common electrode signal, the drivecircuit drives an LC of one of the reflective area and the transmissivearea based on a common pixel signal that is common to the reflectivearea and the transmissive area, and based on the first common electrodesignal, a voltage characteristic, Vr(K)−Vr, of the reflective area issubstantially equivalent to a second voltage characteristic, Vt−Vt(K),of the transmissive area when a first drive voltage Vr and a seconddrive voltage Vt change by a voltage ΔV, the Vr and Vt are drivevoltages of the LC in the reflective area and the transmissive area,respectively, Vr(K) is a dark-state setup voltage in the reflectivearea, Vt(K) is a dark-state setup voltage in the transmissive area, R isa reflectance, and T is a transmittance, The dark-state setup voltage isselected according to a comb teeth clearance, a first common electrodeand a first pixel electrode in said reflective area are comb teethelectrodes and are disposed in such a manner that teeth of said firstcommon electrode and teeth of said first pixel electrode are alternatelyaligned with a clearance Lr in an alignment direction, a second commonelectrode and a second pixel electrode in said transmissive area arecomb teeth electrodes and are disposed in such a manner that teeth ofthe second common electrode and teeth of the second pixel electrode arealternately aligned with a clearance Lt in an alignment directionthereof, and Lt is greater than Lr, wherein α(R) is 25° where α(R) is anangle formed between a direction in which said teeth of said firstcommon electrode and said first pixel electrode in said reflective areaextend and a rubbing direction for said LC, wherein α(T) ranges from 15°to 25° where α(T) is an angle formed between a direction in which saidteeth of said second common electrode and said second pixel electrode insaid transmissive area extend and said rubbing direction for said LC. 2.The transflective LCD device according to claim 1, wherein the secondcommon electrode signal has an amplitude larger than the amplitude ofthe first common electrode signal.
 3. The transflective LCD deviceaccording to claim 1, wherein the Vr (K) that is the dark-state setupvoltage in the reflective area is above
 0. 4. A transflective liquidcrystal display (LCD) device comprising an LCD panel including an arrayof pixels each having a reflective area and a transmissive area in aliquid crystal (LC) layer, and a drive circuit for driving saidreflective area and said transmissive area of said LC layer by using aninverting drive scheme, wherein the drive circuit generates a firstcommon electrode signal periodically inverted in a polarity thereof at apredetermined amplitude, and generates a second common electrode signalhaving a phase opposite to a phase of the first common electrode signal,the drive circuit drives an LC of one of the reflective area and thetransmissive area based on a common pixel signal that is common to thereflective area and the transmissive area, and based on the first commonelectrode signal, the drive circuit drives an LC of the other of thereflective area and the transmissive area based on the common pixelsignal and based on the second common electrode signal, and where Vr andVt are drive voltages of the LC in the reflective area and thetransmissive area, respectively, Vr(K) is a dark-state setup voltage inthe reflective area, VR(W) is a bright-state setup voltage in thereflective area, Vt(K) is a dark-state setup voltage in the transmissivearea, Vt(W) is a bright-state setup voltage in the transmissive area, Ris the reflectance, and T is the transmittance, a slope of a reflectancein vicinity of said Vr(K) in a graph describing R characteristics withrespect to [Vr(K)−Vr] and a slope of a reflectance in vicinity of Vr(W)in the graph match each other, and a slope of a transmittance invicinity of Vt(K) in a graph describing T characteristics with respectto [Vt−Vt(K)] and a slope of a transmittance in vicinity of Vt(W) in thegraph match each other, the dark-state setup voltage is selectedaccording to a comb teeth clearance, a first common electrode and afirst pixel electrode in the reflective area are comb teeth electrodesand are disposed in such a manner that teeth of the first commonelectrode and teeth of the first pixel electrode are alternately alignedwith a clearance Lr in an alignment direction thereof, a second commonelectrode and a second pixel electrode in the transmissive area are combteeth electrodes and are disposed in such a manner that teeth of thesecond common electrode and teeth of the second pixel electrode arealternately aligned with a clearance Lt in an alignment directionthereof, and Lt is greater than Lr, wherein α(R) is 25° where α(R) is anangle formed between a direction in which said teeth of said firstcommon electrode and said first pixel electrode in said reflective areaextend and a rubbing direction for said LC, wherein α(T) ranges from 15°to 25° where α(T) is an angle formed between a direction in which saidteeth of said second common electrode and said second pixel electrode insaid transmissive area extend and said rubbing direction for said LC. 5.The transflective LCD device according to claim 4, wherein the secondcommon electrode signal has an amplitude larger than the amplitude ofthe first common electrode signal.
 6. The transflective LCD deviceaccording to claim 4, wherein the Vr (K) that is the dark-state setupvoltage in the reflective area is above 0.